Continuous optical discharge (COD) is a stationary gas discharge sustained by laser radiation in pre-created relatively dense plasma. A COD, sustained by a focused beam of a continuous wave (CW) laser, is realized in various gases, in particular, in Xe at a high gas pressure of 10-200 atm (Carlhoff et al., “Continuous Optical Discharges at Very High Pressure,” Physica 103C, 1981, pp. 439-447). Due to a high plasma temperature of about 20,000 K (Raizer, “Optical Discharges,” Sov. Phys. Usp. 23 (11), November 1980, pp. 789-806) COD-based light sources are among the highest brightness continuous light sources in a wide spectral range between about 0.1 μm and 1 μm. Compared to arc lamps, such laser-pumped plasma light sources not only have a higher brightness, but also a longer lifetime, making them preferable for a variety of applications.
One of the challenges related to designing high-brightness laser-pumped plasma light sources, relates to producing initial plasma that provides reliable ignition of the COD.
As known, for example, from the U.S. Pat. No. 9,368,337, issued on Jun. 14, 2016, in laser-pumped plasma light source two pin electrodes, located on the axis of a transparent chamber, between which an arc discharge is generated for a short time, are used for starting plasma ignition. The beam of CW laser is focused in the chamber center, in the gap between the two electrodes. The source is characterized by high brightness and ease of use. The latter is largely due to the fact that quartz chambers or bulbs with two electrodes, containing gas, in particular, high-pressure Xe (10 atm or higher), are commercially available products.
However, the relatively cold electrodes located near the high-temperature plasma region produce disturbances of convective gas flows in the chamber and, as a result, impair spatial and energetic stability of the laser-pumped plasma light source. Besides, the presence of electrodes near the radiating plasma region is characterized by “dead” spatial angles restricting the exit of plasma radiation. Also, electrode material sputtering may result in decreased transparency of the bulb walls and, correspondingly, to the light source degradation over time.
This drawback is largely overcome in the high-brightness broadband light source known from the U.S. Pat. No. 9,357,627, issued on May 31, 2016. In its embodiment, after COD ignition the laser beam focus area and, correspondingly, the radiating plasma region, are moved from the gap between the igniting electrodes towards the chamber wall. By choosing the relative position of the laser beam, chamber axis and radiating plasma region, a high spatial and power stability of the broadband laser-pumped plasma light source is provided.
However, the need to move the radiating plasma region complicates the light source design and operation. Besides, it makes using the sharp focusing of the laser beam more difficult, which may limit achievement of the high brightness of the light source. The disadvantages of an electrode-containing chamber also include the complex technology for sealing the metal/glass joint and the complex chamber shape producing a concentration of stresses which result in lower strength of the chamber when operating at high gas pressures.
The above-mentioned disadvantages are absent in the electrode-free laser-pumped plasma light source known from the patent application JPS 61193358 issued on Aug. 27, 1986, where the laser is used both for starting plasma ignition and for COD sustenance.
However, the threshold power of laser radiation required for plasma ignition is usually from about ten to several hundreds of kilowatts or higher, while the laser radiation intensity sufficient for COD sustenance is typically just a few tens of Watts. Thus, using the same laser with a high output power both for plasma ignition and COD sustenance either results in reduced lifetime of the light source (when the full laser power is used for COD sustenance), or is redundant, expensive and, therefore, impractical if only a fraction of the full laser power is used to sustain the COD.
U.S. Pat. No. 10,057,973, issued on Aug. 21, 2018, proposes to overcome this challenge by using a single CW laser with the power of less than 250 Watts and a wavelength of less than 1.1 μm. It is suggested that COD ignition and sustenance is provided by means of sharp focusing of CW laser beam with a focal area cross size of less than 1-15 microns, and a focal area length of 6 microns or lower.
However, this solution is not versatile, since the requirements to laser focusing are very high and do not guarantee high functional reliability of the proposed light source. Besides, the laser power of around 250 Watts supplied to the light source may be too high for a variety of applications.
These disadvantages are overcome in the light source known from patent FR2554302 issued on May 3, 1985, where a focused pulsed laser beam intended for initial plasma ignition or optical breakdown is used as a means for plasma ignition and a CW laser is used for COD sustenance. The above-mentioned approach eliminates the problem of the laser-pumped plasma light source lifetime.
However, sharp focusing of laser beams is required for both plasma ignition and ensuring high brightness of laser-pumped plasma light source. Thus, an extremely precise adjustment is needed of the pulsed and CW laser focusing areas. This results in a complexity and poor reliability of laser ignition, making stable COD ignition in a high-brightness light source problematic.
These disadvantages are partially overcome in the light source known from the U.S. Pat. No. 10,244,613, issued on May 25, 2017. In an embodiment of the invention the beams of one or several igniting lasers and beams of one or several CW lasers, intended for COD sustenance, are introduced into an optical fiber used for delivering the radiation of the said lasers to a condensing or focusing optical system. In the said device, superposition of the focusing areas of the pulsed lasers and of CW lasers is achieved, if the wavelengths of the said lasers are similar.
However, if the pulsed and CW laser wavelengths are different, their focusing areas diverge due to chromatic aberrations. Besides, transmitting laser pulses with the high power used for reliable COD ignition (hundreds of kW) through an optical fiber may result in optical fiber destruction, which determines the disadvantages of this solution.